Inter-cell interference coordination (ICIC) was introduced in Release-8/9 of the 3GPP LTE standards. The basic idea of ICIC is keeping the inter-cell interferences under control by radio resource management (RRM) methods. ICIC is inherently a multi-cell RRM function that needs to take into account information (e.g. the resource usage status and traffic load situation) from multiple cells. Broadly speaking, the main target of any ICIC strategy is to determine the resources (bandwidth and power) at each cell at any time. Then (and typically), a scheduler assigns those resources to users. Static ICIC schemes are attractive for operators since the complexity of their deployment is very low and there is no need for new extra signaling out of the standard. Static ICIC mostly relies on the fractional frequency reuse concept, where the total system bandwidth is divided into sub-bands and used by the scheduler accordingly.
LTE Release-8/9 ICIC techniques, however, are not fully effective in mitigating control channel interference. For example, dominant interference condition has been shown when non-CSG (close subscriber group) macrocell users are in close proximity of CSG femtocells. Therefore, enhanced ICIC (eICIC) has been investigated from Release-10 onwards to provide enhanced interference management. In LTE/LTE-A Release-10, two main inter-cell interference scenarios for eICIC were being discussed: Macro-Pico scenario and Macro-Femto scenario. In general, almost-blank subframe (ABS) or silenced subframe concept is introduced to reduce inter-cell interference. When ABS is applied, the aggressor cell suspends the scheduling or transmits with smaller power so that the victim cell can conduct data transmission in the protected subframes.
In Macro-Pico scenario, a macrocell is the aggressor and may introduce strong interferences to picocells, which are called victim cells. In this scenario, macrocell UEs operate typically in connected mode. ABS is applied in the macrocell so that UEs can try to search for picocells in the protected subframes. Several radio resource management (RRM) technologies are available in LTE/LTE-A systems to mitigate inter-cell interference. In one RRM scheme, a UE may declare radio link failure (RLF) based on radio link monitoring (RLM) measurements. Another possible RRM scheme is that the UE may report measurement results to its serving base station (eNB) for better scheduling and mobility management. Since only some subframes are protected in picocell, such measurements should be modified accordingly. Otherwise, the measurement results would be largely affected by the interfering macrocell.
In Macro-Femto scenario, a non-accessible CSG femtocell is the interferer and macrocell is the victim cell, and macrocell UEs may be in connected mode or in idle mode. ABS is applied in the femtocell. The current LTE RRM design has not investigated eICIC for idle mode. However, for the case of Macro-Femto inter-cell interference, UEs in idle mode also need interference coordination to prevent from any cell selection and go out-of-service (OOS) in cases when no alternative carrier is available. For example, when a UE connected to a macrocell moves into the vicinity of a non-accessible CSG femtocell, the UE can stay connected to the macrocell thanks to inter-cell interference coordination. When the UE goes to idle mode later on, UE measurements will indicate that the macrocell is no longer suitable and the UE goes to out of service. Without interference coordination, the UE in idle mode cannot return to connected mode, unless the UE moves out of the interfering of the femtocell. Therefore, UE measurements adapted to interference coordination is desirable for UEs in idle mode.
In the presence of strong inter-cell interference, it is also desirable that enhanced network access procedure such as random access channel (RACH) procedure can be applied to improve interference coordination. In addition, for UEs in connected mode, UE measurement enhancements are also needed to increase radio spectrum efficiency and to improve user experiences.